Research On Integrated Global/Local Structural Dynamic Model Updating Method

The structural dynamic strength analysis is not often taken into account in the engineering practice of structure design.Many structural failure problems caused by excessive dynamic stress or vibration fatigue occurred in services especially in aerospace engineering.Therefore,special attentions should be paid to the simulative evaluation of structural dynamic strength in structural design.It is well known that the reasonable dynamic finite element(FE)model is the prerequisite for performing accurate structural dynamic analysis and dynamic strength evaluation.Though the dynamic model updated by the conventional FE updating methods can accurately characterize the global dynamic characteristics of the structure,such as the natural frequency,it is difficult to describe the dynamic stress-strain characteristics of the local structure.In order to improve the accuracy for calculating the dynamic strains and their transfer relations of the local structure,an integrated global/local structural dynamic model updating method based on dynamic strains is investigated in the present research to overcome the shortage in model updating for the critical local sites of the structure.In this thesis,the researchs of the structural dynamic FE model updating method are expatiated in three aspects,i.e.updating parameters,objective function and algorithm used in updating method.Then,the key problems to be solved in this thesis are illustrated as follows: 1)how to select updating parameters;2)how to propose a reasonable extraction method and a good matching criterion of structural dynamic characteristic parameters;3)how to establish a reasonable model updating algorithm.The research work in this thesis aims to solve the above problems.The strain mode has the characteristics of traditional displacement mode,and can reflect the local dynamic stress-strain characteristics of the structure.It is also sensitive to local changes in the structure,such as changes in geometrical,physical and mechanical parameters.The strain mode,served as the generalized modal parameter,can be identified by the measured strain frequency response function data.Then,a model updating method based on strain modal residuals using hybrid pattern search technique is proposed.In this updating method,the objective function for model updating is established by means of strain mode residuals.Moreover,the hybrid pattern search(HPS)optimization technique,which synthetizes the advantages of pattern search(PS)optimization technique and genetic algorithm(GA),is introduced to solve the dynamic FE model updating problem.Numerical simulation and model updating experiment for a clamped-clamped beam are performed to validate the feasibility and effectiveness of the present method.The updated results show that the proposed method can be successfully used to modify the incorrect parameters with good robustness.It is well known that the structure information included in strain frequency response function data is more than those in strain modal parameters.And in order to ensure that the updated dynamic FE model can better reflect the dynamic characteristics and the dynamic stress-strain characteristics for the critical local sites of the structure in the concerned band.An updating method is proposed to modify the structural parameters of a dynamic FE model by using the correlation analysis for strain frequency response function.By means of the correlation analysis between experimental and analytical SFRFs at the specific critical locations of a structure,a dynamic FE model updating method is proposed.A frequency selection strategy is also introduced to improve the computational efficiency and the stability of the proposed updating method.Based on sensitivity analysis of the correlation coefficients between the experimental and analytical SFRFs,the linear algebraic equations for model updating are established to modify the modeling errors.Meanwhile,the ill-pose problem in updating procedure is solved by regularization technique.Finally,a numerical study and a model updating experiment are performed to verify the feasibility and robustness of the proposed method.The results show that the updated SFRFs and experimental SFRFs agree well,especially in the resonance regions.Meanwhile,the proposed method has good robustness to noise and remains good feasibility even the number of measurement locations reduced significantly.As the structural failure without the dynamic strength checking is a local failure mode,the critical local structure can only be considered to updating while ensuring the dynamic characteristic of the whole structure is accurate.This can improve the model updating efficiency.Then,a local structural model updating method considering the global dynamic characteristics as constraints is proposed in this thesis.By using the idea of “separation-equivalence”,the local structure to be updated is separated from the global structure.Meanwhile,all the surrounding structure is equivalent to the dynamic boundary of the local structure.Then,the SFRF is used as the characteristic for formulating the objective function of the model updating method,and a model updating of “layer-iterative” is proposed.At last,a numerical simulation and an experiment for model updating of a cantilever plate with elastic support are performed to validate the feasibility and effectiveness of the proposed method.The results show that,the proposed method can be successfully used to update the dynamic FE model of the local structure,and there is a good agreement between the SFRFs of the updated FE model and the real structure.Finally,the research work and innovation aspect of the dissertation are summarized.Considering the progress and demands of investigation in structural model updating,the future work of this dissertation is also proposed.